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1
Dough: Stir together dry ingredients in the bottom of a bowl. In a bowl, whisk together egg, oil, vinegar and water. Pour it over dry ingredients and stir to combine. Once mixture is an uneven mass, knead it until smooth. Place dough back in the bowl and cover with plastic wrap. Set it aside for an hour until needed.
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2
Prepare filling: Put potatoes in a large pot, cover with cold water, and bring to a boil over high heat. Reduce heat to medium and cook until potatoes can be pierced easily with a knife, about 20 minutes. Drain, then transfer to a large bowl.
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3
Heat a large skillet over medium heat. Once hot, add butter and oil and once they're fully melted add onions and reduce to medium-low. Cook, stirring frequently, until deeply caramelized, about 45 minutes. Transfer to bowl with potatoes and mash together until almost smooth. Stir in salt and many grinds of black pepper and set the filling aside.
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4
Assemble knish: Preheat your oven to 375.
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5
Divide the dough in half. On a well-floured surface, roll the first half of the dough into a very thin sheet, roughly in the shape of a 1-foot square. Create a 2-inch thick log from half your potato filling across the bottom of your dough. Roll the filling up in the dough, but not too tight. Keep rolling until the log has been wrapped twice in dough. Repeat the process with the second half of your dough and filling.
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6
Trim the ends of the dough so that they're even with the potato filling. Make indentations on the log every 3 inches and twist the dough at these points. Snip the dough at each twist, then pinch one of the ends of each segment together to form a sealed knish base. Use the palm of your hand to flatten the knish a bit into a squat shape. Pinch together the tops as you did the bottom to seal them; indenting them with a small dimple will help keep them from opening in the oven.
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7
Bake knish: Whisk egg yolk and water together to form a glaze and brush it over the knish dough. Bake knish for about 45 minutes.
